Conventional cathode ray tubes (CRTs) are used in display monitors for computers, television sets, and other video devices to visually display information. Use of a luminescent phosphor coating on a transparent face, such as glass, allows the CRT to communicate qualities such as color, brightness, contrast and resolution which, together, form a picture for the benefit of a viewer.
Conventional CRTs have, among other things, the disadvantage of requiring significant physical depth, i.e. space behind the actual display screen, resulting in such units being large and cumbersome. There are a number of important applications in which this physical depth is deleterious. For example, the depth available for many compact portable computer displays precludes the use of conventional CRTs. Furthermore, portable computers cannot tolerate the additional weight and power consumption of conventional CRTs. To overcome these disadvantages, displays have been developed which do not have the depth, weight or power consumption of conventional CRTs. These "flat panel" displays have thus far been designed to use technologies such as passive or active matrix liquid crystal displays ("LCD") or electroluminescent ("EL") or gas plasma displays.
A flat panel display fills the void left by conventional CRTs. However, the flat panel displays based on liquid crystal technology either produce a picture which is degraded in its fidelity or is non-emissive. Some liquid crystal displays have overcome the non-emissiveness problem by providing a backlight, but this has its own disadvantage of requiring more energy. Since portable computers typically operate on limited battery power, this becomes an extreme disadvantage. The performance of passive matrix LCD may be improved by using active matrix LCD technology, but the manufacturing yield of such displays is very low due to required complex processing controls and tight tolerances. EL and gas plasma displays are brighter and more readable than liquid crystal displays, but are more expensive and require a significant amount of energy to operate.
Field emission displays combine the visual display advantages of the conventional CRT with the depth, weight and power consumption advantages of more conventional flat panel liquid crystal, EL and gas plasma displays. Such field emission displays use very sharp micro-tips made of tungsten, molybdenum or silicon as the cold electron emitter. Electrons emitted from the cathode due to the presence of an electric field applied between the cathode and the grid bombard the phosphor anode, thereby generating light.
Such a matrix-addressed flat panel display is taught in U.S. Pat. No. 5,015,912, which issued on May 14, 1991, to Spindt et al., and which uses microtip cathodes of the field emission type. The cathodes are incorporated into the display backing structure, and energize corresponding cathodoluminescent areas on a face plate. The face plate is spaced 40 microns from the cathode arrangement in the preferred embodiment, and a vacuum is provided in the space between the plate and cathodes. Spacers in the form of legs interspersed among the pixels maintain the spacing, and electrical connections for the bases of the cathodes are diffused sections through the backing structure.
An attribute of the invention disclosed in Spindt et al. is that it provides its matrix-addressing scheme entirely within the cathode assembly. Each cathode includes a multitude of spaced-apart electron emitting tips which project upwardly therefrom toward the face structure. An electrically conductive gate or extraction electrode arrangement is positioned adjacent the tips to generate and control electron emission from the latter. Such arrangement is perpendicular to the base stripes and includes apertures through which electrons emitted by the tips may pass. The extraction electrode is addressed in conjunction with selected individual cathodes to produce emission from the selected individual cathodes. The grid-cathode arrangement is necessary in micro-tip cathodes constructed of tungsten, molybdenum or silicon, because the extraction field necessary to cause emission of electrons exceeds 50 Megavolts per meter ("MV/m"). Thus, the grid must be placed close (within approximately 1 micrometer) to the micro-tip cathodes. These tight tolerances require that the gate electrodes be produced by optical lithographic techniques on an electrical insulating layer which electrically separates the gates of each pixel from the common base. Such photolithography is expensive and difficult to accomplish with the accuracy required to produce such a display, thereby raising rejection rates for completed displays.
The two major problems with the device disclosed in Spindt et al. are 1) formation of the micro-tip cathodes and 2) formation and alignment of the extraction electrodes with respect to the cathodes. The structure disclosed in Spindt et al. is extremely intricate and difficult to fabricate in the case of large area displays. Thus, the invention disclosed in Spindt et al. does not address the need for a flat panel display which is less complicated and less expensive to manufacture.
The above-mentioned problems may be alleviated if the grid structure and sharp micro-tips are not needed. This may be accomplished by use of a flat cathode as the electron field emitter in a diode configuration where the anode is coated with a phosphor. No extraction grid is needed in such a display, thereby rendering the display relatively easy to construct.
Unfortunately, such field emission flat panel displays having a diode (cathode/anode) configuration suffer from several disadvantages.
First, the energy of electrons bombarding phosphors coating the anode is determined by the voltage between the cathode and the phosphors on the anode. In color displays, in which the phosphors must be excited by an especially high electron energy, cathode/anode voltage should be higher than 300 volts. This high voltage requirement causes cathode and anode drivers to be able to handle the higher voltage, thus making the drivers more expensive to manufacture. Such high voltage drivers are also relatively slow due to the time it takes to develop the higher voltage on conductors within the display.
According to Fowler-Nordheim ("F-N") theory, the current density of field emissions changes by as much as 10 percent when cathode/anode separation changes by only 1 percent. Prior art flat panel displays have not been completely successful in overcoming the problem of field emission variations.
All flat panel displays must employ an addressing scheme of some sort to allow information a computer or other device sends to the display to be placed in proper order. Addressing is simply the means by which individual display or picture elements (frequently called "pixels") are accessed and configured to display the information.
A related issue which must be addressed in the context of flat panel displays is proper spacing between anode and cathode assemblies. As has been discussed, proper spacing is critical in controlling field emission variation from one pixel to another and in minimizing the voltage required to drive the display. In triode displays, glass balls, fibers, polyimides and other insulators have been used to maintain proper separation. In such displays, separation is not as critical because the electric field between the anode and electron extraction grid is not as great (on the order of 10%) of the electric field between the grid and the cathode (the electron extraction field). In diode displays, a spacer must have a breakdown electric field much larger than the electron extraction field for the cathode.
To be useful in today's computer and video markets, flat panel displays must be able to create pictures having greys (half-tones) thereby allowing the displays to create graphical images in addition to textual images. In the past, both analog and duty-cycle modulation techniques have been used to implement grey-scale operation of a flat panel display.
The first of these is analog control. By varying voltage in a continuous fashion, individual pixels thus excited can be driven to variable intensities, allowing grey-scale operation. The second of these is duty-cycle modulation. One of the most often employed versions of this type of control is that of pulse-width modulation, in which a given pixel is either completely "on" or completely "off" at a given time, but the pixel is so rapidly switched between the "on" and "off" states that the pixel appears to assume a state between "on" and "off." If the dwell times in the "on" or "off" states are made unequal, the pixel can be made to assume any one of a number of grey states between black and white. Both of these methods are useful in controlling diode displays.
A matrix-addressable flat panel display which is simple and relatively inexpensive to manufacture and which incorporates redundancy for continued operation of each pixel within the display is required to overcome the above-noted disadvantages. The display should embody a sophisticated cathode/anode spacing scheme which is nonetheless reliable and inexpensive to manufacture. Finally, the display should also embody a scheme for implementing a grey scale mode within a flat panel display of diode pixel structure to allow individual pixels to assume shades between black and white, thereby increasing the information-carrying capacity and versatility of the display.